1. Field of the Invention
The apparatus and methods consistent with the present invention generally relate to orthogonal frequency division multiplexing (OFDM) communications systems and, more particularly, to achieving faster timing synchronization in an OFDM receiver.
2 Description of the Prior Art
A. Wireless Networks
A wireless local area network (LAN) typically uses infrared (IR) or radio frequency (RF) communications channels to communicate between portable or mobile computer terminals and stationary access points or base stations. These access points are, in turn, connected by a wired or wireless communications channel to a network infrastructure which connects groups of access points together to form the LAN, including, optionally, one or more host computer systems.
Wireless IR and RF protocols are known which support the logical interconnections of such portable roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some of the terminals are capable of communicating with at least two of the access points when located within a predetermined range therefrom, each terminal being normally associated, and in communication, with a single one of such access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications.
One such protocol is described in U.S. Pat. Nos. 5,029,183; 5,142,550; 5,280,498; and 5,668,803, each assigned to the assignee of this application and incorporated herein by reference. Still another protocol is set forth in the IEEE Standard 802.11 entitled xe2x80x9cWireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specificationsxe2x80x9d available from the IEEE Standards Department, Piscataway, N.J. (hereinafter, the xe2x80x9cIEEE 802.11 Standardxe2x80x9d).
IEEE Project 802 is concerned with network architecture for LANs. The IEEE 802.11 Standard is directed to wireless LANs, and in particular specifies the MAC and the PHY layers. These layers are intended to correspond closely to the two lowest layers of the ISO Basic Reference Model of OSI, i.e., the data link layer and the physical layer.
The IEEE 802.11 Standard permits either IR or RF communications at 1 Mbps, 2 Mbps and higher data rates, a medium access technique similar to carrier sense multiple access/collision avoidance (CSMA/CA), a power-save mode for battery-operated mobile stations, seamless roaming in a full cellular network, high throughput operation, diverse antenna systems designed to eliminate xe2x80x9cdead spotsxe2x80x9d, and an easy interface to existing network infrastructures.
In Europe, the European Telecommunications Standards Institute (ETSI) has been working on HIPERLAN (European HIgh PERformance LAN), the next generation of high speed wireless systems. The frequency spectrum for HIPERLAN in the 5 GHz and 17 GHz bands has been allocated by the European Conference of Postal and Telecommunications Administrations (CEPT), with a data rate of over 20 Mbit/sec.
B. Spread Spectrum Modulation Techniques
The current implementations of commercial wireless LANs utilize a radio operating in the 2.4 to 2.4835 GHz spread spectrum band which is the industrial, scientific, and medical (ISM) band allocated for unlicenced use by the Federal Communications Commission (FCC). The current systems utilize one of two basic types of spread spectrum modulation: direct-sequence and frequency-hopping. In the description that follows, the specific modulation parameters specified by the IEEE 802.11 Standard shall be used to illustrate the different modulation techniques.
In a direct-sequence spread spectrum (DSSS) system, each binary bit of data in a data signal is spread over each of 11 discrete frequency channels at the same time, i.e., an 11-bit pseudo-random noise (PN) code. The data of each user is coded using a different PN code so that the signals of different users are orthogonal to each other. Thus, another user""s signal is merely interpreted as noise. The IEEE 802.11 Standard provides two modulation formats and data rates in the DSSS systemxe2x80x94a basic access rate using differential binary phase shift keying (DBPSK) modulation operating at 1 Mbps, and an enhanced access rate using differential quadrature phase shift keying (DQPSK) modulation operating at 2 Mbps.
In a frequency-hopping spread spectrum (FHSS) system, each binary bit of data in the data signal is associated with a group of distinct xe2x80x9cchipsxe2x80x9d, or discrete signal frequency output, in different parts of a frequency band, with a minimum hop of at least 6 MHZ (in North America/Europe). The chipping pattern or hopping sequence is a pseudo-random sequence uniformly distributed throughout the band and set forth in the IEEE 802.11 Standard. Each access point executes a unique hopping pattern across 79 non-overlapping frequencies at a rate of one hop every 100 milliseconds. There are three sets of hopping patterns specified in the IEEE 802.11 Standard for North American/European operations, with each set containing 26 sequences. The sets are selected to minimize the possibility of interference. The RF modulation technique used in the FHSS system is 2-level or 4-level Gaussian filtered frequency shift keying (GFSK). Frequency-hopping spread spectrum systems are currently preferred over direct-sequence for most applications by the majority of users as they allow increased capacity and decreased interference.
The IEEE 802.11 FHSS systems hop over channels with an effective raw data rate of 1 Mbps or 2 Mbps. Current commercial systems can typically cover from an area of 25,000 to 70,000 square feet with a process gain of 10 dB. The relatively low power output used in such systems is a consequence of limits placed by regulatory agencies. Power output standards currently in effect limit the power output to either 100 mW, 230 mW, or 500 mW depending on the country.
In a spread spectrum system, one can multiplex users by assigning them different spreading keys. Such a system is called a code division multiple access (CDMA) system. Most wireless LANs are not CDMA systems since users belonging to the same wireless LAN utilize the same spreading key. Instead, as noted above, the MAC set forth in the IEEE 802.11 Standard provides that use access to the channel is multiplexed in time using nearly the same Carrier Sense Multiple Access (CSMA) protocol as in the Ethernet.
The CDMA modulation technique is one of several techniques for facilitating communications in which a large number of system users is present. The use of CDMA in a digital cellular spread spectrum communications system was adopted by the Telecommunication Industry Association in 1993 as standard IS-95. Other multiple access communications system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307.
C. OFDM Communications Systems
The IEEE 802.1a Standard also specifies the PHY layer operating in the 5 GHz band, which is open to unlicensed devices in the U.S. The IEEE 802.11a Standard is based on orthogonal frequency division multiplexing (OFDM) to modulate the data. Digital data is divided among a large number of adjacent carriers so that a relatively small amount of data is carried on each carrier. Adjacent carriers are mathematically orthogonal. Their sidebands may overlap but signals can be received without adjacent carrier interference. The main benefit of OFDM modulation is its robustness to multipath echoes, which are encountered in the indoor and mobile environments. Each OFDM symbol is composed of fifty-two non-zero subcarriers of which forty-eight are data subcarriers and the remaining four are carrier pilot subcarriers. The PHY specifications encompass data rates from 6 Mbit/s up to 54 Mbit/s, with 20 MHZ spacing between adjacent channels. All implementations are required to support 6, 12 and 24 Mbit/s. Optional extensions are for 9, 18, 36, 48 and 54 Mbit/s. The range of data rates is provided to match the wide range of radio channel characteristics in both indoor and outdoor environments. The multirate mechanism of the MAC protocol ensures that all devices communicate with each other at the best data rate in the present channel.
In a conventional single-carrier digital communication system, data symbols are transmitted serially using some modulation scheme, and the spectrum of each symbol is allowed to occupy the entire channel bandwidth. In multicarrier modulation schemes, data symbols are transmitted in parallel on multiple subcarriers that share the channel bandwidth using some form of frequency-division multiplexing (FDM). The modulation scheme on one subcarrier may be chosen independent of that used on other subcarriers. Thus, subcarriers in frequency segments of the channel with a high signal-to-noise ratio (SNR) may use high-rate modulation, while those with degraded SNR use low-rate modulation, or are not modulated. Systems which adaptively load the subcarriers differently depending on the spectral shaping of the channel are common in wired applications such as asymmetric digital subscriber lines (ADSL), and the technique is usually referred to as discrete multitone or DMT. DMT systems have been widely analyzed and reported in the prior art.
In OFDM the spectra of the subcarriers overlap, and their spacing is chosen so that each subcarrier is orthogonal to all other subcarriers. The common method of obtaining orthogonality of subcarriers is to choose their frequency spacing equal to the inverse of the subcarrier symbol duration. Baseband processing of the OFDM signal is then conveniently effected using the discrete Fourier transform, implemented as an inverse fast Fourier transform (IFFT) and a fast Fourier transform (FFT) that modulate and demodulate parallel data blocks, respectively. The set of subcarriers generated during one transform defines an OFDM symbol. The subcarriers are conveyed by serial transmission over the channel of the time samples generated by the IFFT. The duration of the OFDM symbol, which is the same as that of the subcarrier symbol, is thus equal to the time window of the transform.
Various techniques for compensating for local oscillator errors in an OFDM system are described in the known art. For example, U.S. Pat. No. 5,838,734 describes a receiver in which the phase errors in a received OFDM signal are analyzed and corrected phase values derived. In particular, U.S. Pat. No. 5,838,734 discloses an FFT with outputs for the I and Q values of each of the carriers which were originally encoded at the transmitter. These pass to a converter which derives the magnitude Z for each vector from the quadrature amplitude modulation (QAM) phase diagram which they represent. These I and Q values also pass to a converter which derives an angle for each vector in the QAM phase diagram and supplies this to a phase error analyzer as well as to a phase error compensator. The phase error analyzer removes phase noise due to the local oscillator, and the phase angles are then corrected in the phase error compensator to provide a corrected output.
An OFDM signal includes a preamble followed by a signal symbol and a variable number of data symbols. The preamble includes ten so-called xe2x80x9cshortxe2x80x9d symbols (for example, each of duration t=0.8 xcexcs), followed by a single so-called xe2x80x9cmediumxe2x80x9d symbol (for example, of duration t=1.6 xcexcs), followed by two so-called xe2x80x9clongxe2x80x9d symbols (for example, each of duration t=3.2 xcexcs).
When the OFDM signal is received, various functions are performed during the receipt of the symbols in the preamble. One of these functions is timing synchronization and is the main function with which the instant invention is concerned.
In the prior art, two of the short symbols are correlated to obtain a peak whose time of occurrence is compared to a reference time, thereby establishing timing information. However, the use of two short symbols for obtaining timing information is too much time for some applications. For example, some OFDM receivers have two antennas, and it is desired to select one for the OFDM signal reception. Typically, for the first antenna, one short symbol is used for automatic gain control, another short symbol is used for buffering, and two more short symbols are used for the timing. The same usage of symbols is needed for the second antenna, and still another short symbol is needed for selection between the antennas. There are very few short symbols left in the preamble and, hence, little time to repeat any of the above functions, especially in the event of bad channel selection.
It is a general object of the present invention to provide an improved communications receiver for use in orthogonal frequency division multiplexing communications systems.
It is another object of the invention to provide optimum timing synchronization in an OFDM communications receiver.
It is a further object of the present invention to provide fast antenna selection in an OFDM communications receiver.
It is an even further object of the invention to provide an arrangement and methods which can be used to accomplish one or more of the above objectives.
Additional objects, advantages and novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, as well as by practice of the invention. While the invention is described below with reference to preferred embodiments, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility.
Briefly, and in general terms, the invention relates to a method of synchronizing a receiver to a received orthogonal frequency division multiplexing (OFDM) signal having a preamble with short and long duration symbols, comprising the steps of converting the received OFDM signal by fast Fourier transform to frequency domain components; processing the frequency domain components to timing components; deriving timing information from the timing components; applying the timing information to the received OFDM signal to synchronize the receiver; and all of said steps being performed during one of the short symbols.
Another feature of this invention is embodied in a method of antenna selection in a dual antenna receiver for receiving an orthogonal frequency division multiplexing (OFDM) signal having a preamble with short and long duration symbols, comprising the steps of converting an OFDM signal received by a first antenna by fast Fourier transform to frequency domain components; processing the frequency domain components to timing components; deriving timing information from the timing components; applying the timing information to the received OFDM signal to synchronize the receiver; analyzing a signal-to-noise ratio (SNR) of the OFDM signal received by the first antenna of the synchronized receiver during one of the short symbols; repeating the aforementioned steps for a second antenna during another of the short symbols; and selecting the antenna which has a higher SNR during another of the short symbols.
Yet another feature of this invention resides in an arrangement for synchronizing a receiver to a received orthogonal frequency division multiplexing (OFDM) signal having a preamble with short and long duration symbols, comprising a Fourier transform (FT) circuit for converting the received OFDM signal to frequency domain components; a constellation processing (CP) circuit connected to the FT circuit, for processing the frequency domain components to timing components; and a timing synchronizing circuit connected to the CP circuit, and operative for deriving timing information from the timing components, and for applying the timing information to the received OFDM signal to synchronize the receiver during one of the short symbols.
The novel features and characteristics of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of a specific embodiment, when read in conjunction with the accompanying drawings.